1. Field of the Invention
The present invention relates to refractory articles in general having improved surface properties. In particular, it relates to refractory articles comprised of a fused silica matrix such as rollers for conveying hot articles or articles used for the treatment of molten glass or non-ferrous metals such as crucibles for melting or crystallizing silicon, rotary degasser for the treatment of aluminum, etc.
2. Description of the Related Art
Refractory materials are widely used in highly demanding applications such as the treatment of molten metals or the handling of hot articles. In particular, materials comprised of a fused silica matrix are well known for their excellent behavior at high temperature such as their high refractoriness and low thermal expansion and for their relative inertness with respect to glass or some molten metals.
Certain surface properties of the fused silica materials however make them unsuitable for some applications. In particular, the “surface pickup” behavior (inclination of the material to pick up impurities or fragments present at the surface of the transported article) or the “surface buildup” behavior (inclination of the surface to receive and accumulate metallic particles (such as tin, silicon, etc.) possibly oxidized (tin oxide)) renders quite problematic their use as glass annealing rolls (LOR, Lehr), as glass tempering rolls (GTR) or as containers for handling and/or treating molten glass or metal (e.g. crucible for melting silicon). Marking of the transported articles and adhesion of the silicon crystal to the walls of the crucible are examples of such problems. For such applications, it is necessary either to provide the silica matrix with an expensive and elaborated coating or to use another material (such as metal).
Document GB 2 154 228 A relates to a composition for spraying against a surface to form a refractory mass in situ. Such composition is used for instance to repair the basic refractory blocks of a glass melting furnace. Document U.S. Pat. No. 4,230,498 relates to a patching compositions for refractory furnace blocks in particular silica blocks. The patching composition should bond to the hot silica blocks to which it is applied. Such compositions are used for repairing the walls of furnaces or coke ovens at high temperature. In both cases, the sprayed or patched material is added on top of the existing blocks surface so that the final surface is rough and hardly controllable.
Document FR 2 707 084 relates to an anti corrosive coating for vitreous silica refractory for molten metal applications. Sintering aids are added so that the coating sinters at temperature lower than 1200° C.; 1200° C. being detrimental for the vitreous silica substrate. According to the examples, a 1 mm coating is formed on top of the article surface.
Document U.S. Pat. No. 4,528,244 relates to a SiAlON coating applied on fused silica ware. SiAlON also presents some drawbacks. The glassy phases present in the SiAlON material imply possible gluing or sticking to the substrate. Furthermore, possible oxidation in temperature for the SiAlON and irreversible crystallization of the fused silica makes the SiAlON material unsuitable for thermal cycling applications.
In addition, the provision of a coating is always expensive since it requires additional production steps and further, is only a temporary solution since, unavoidably, the further layer of the coating material has a tendency to delaminate from the surface of the silica matrix material which not only deprives the fused silica material from the benefit of the coating but in turn produces scaling defects. For conveying applications or solid contact applications, since the coating is added on the top of the surface, it is often necessary to subject the coated article to a machining step after the coating step in order to homogenize the surface. Further, since the thermal expansion (or the thermal behavior in general) of this further layer is different from the thermal expansion (or the thermal behavior in general) of the silica matrix, constraints are generated at the interface between the coating and the silica matrix, affecting thereby negatively the surface quality. Moreover, since the thermal conductivity of this further layer is not the same as the thermal conductivity of the fused silica matrix, the transfer of heat through the article is not homogeneous. The use of metallic materials for such articles is also unsatisfactory since metals have a relatively high reactivity towards the treated or transported articles. In addition, metallic articles have a quite high flexural deformation with comparison to fused silica articles and their efficient time of use in a heavy environment (such as a Lehr or a crystallization furnace) is relatively short.